Generally, a mixer circuit for mixing a base-band signal with a local oscillation signal is used for generating a radio frequency (RF) signal, in a RF transmitter.
If frequencies of a base-band signal and a local oscillation signal are IF, LO, respectively, a signal that passes the mixer circuit includes frequency components of IF+LO and IF−LO.
For example, if a frequency of a signal that would be transmitted by the transmitter is IF−LO, the frequency component of IF+LO needs to be rejected. In the specification, the frequency component that would be rejected is called as spurious.
For spurious rejection, a band pass filter that only passes frequency component of IF−LO is used in an output terminal of a mixer, or a low pass filter that passes frequency components higher than IF−LO is used.
However, rejecting the frequency component of IF+LO that is not wanted to transmit perfectly is impossible, though characteristic of the band pass filter or the low pass filter is excellent. Moreover, using a filter in a transmitter is not rational, because price of a band pass filter or a low pass filter that is matched with high frequency signal is generally high.
Accordingly, a quadrature modulation mixer circuit is used as a mixer of a transmitter.
FIG. 1 shows a block diagram of a conventional quadrature modulation mixer that is used in a transmitter.
As shown in FIG. 1, an in-phase component I of a base-band signal is inputted to an in-phase input terminal of the quadrature modulation mixer, and a quadrature-phase component Q of the base-band signal whose phase is delayed from phase of the in-phase component I by 90 degree is inputted to a quadrature-phase input terminal. An in-phase and a quadrature-phase components II and QQ of a high frequency signal are generated as the in-phase and the quadrature-phase components I and Q of the base-band signal are mixed with an in-phase and a quadrature-phase components LOI and LOQ of local signal, respectively, in an in-phase mixer (101) and a quadrature-phase mixer (103). Spurious is rejected according as high frequency signals II and QQ are summed in the output terminal.
Sometimes, a quadrature modulating structure of two stages is taken for improving performance of a mixer circuit in transmitting terminal. FIG. 2 shows a block diagram of a quadrature modulating structure of two stages.
As shown in FIG. 2, a quadrature modulating mixer consists of an IF mixing unit (201) for mixing a base-band signal with a local signal LO1 having intermediate frequency, and a RF mixing unit (203) for mixing an output signal from the IF mixing unit (201) with local signal LO2 having a high frequency. Generally, a base-band signal is a digital signal, and the IF mixing unit (203) for processing the base-band signal is embodied to a digital circuit. Moreover, the output signal from the IF mixing unit (201) is converted to an analog signal by a digital to analog converter DAT. Generally, the RF mixing unit (203) that processes an intermediate frequency signal converted to an analog signal is embodied as an analog circuit.
The IF mixing unit (201) is divided into an in-phase path where the in-phase component I of the base-band signal is mixed with an in-phase and a quadrature-phase components LO1I and LO1Q of the local signal LO1 having intermediate frequency, and a quadrature-phase path where the quadrature-phase component Q of the base-band signal is mixed with the in-phase and the quadrature-phase components LO1I and LO1Q of the local signal LO1 having intermediate frequency.
The signal that passes the in-phase path of the IF mixing unit (201) comprises an in-phase and a quadrature-phase components II and IQ having intermediate frequency. Moreover, the signal that passes the quadrature-phase path of the IF mixing unit (201) comprises an in-phase and a quadrature-phase components QI and QQ having intermediate frequency.
The output signal from the IF mixing unit (201) is converted to a digital signal by a digital to analog converter. In this case, a converted analog signal of the signal that passes the in-phase path of the IF mixing unit (201) is called as an in-phase component IF-I having intermediate frequency, and a converted analog signal of the signal that passes the quadrature-phase path of the IF mixing unit (201) is called as a quadrature-phase component IF-Q having intermediate frequency.
The RF mixing unit (203) is divided into an in-phase path where the in-phase component IF-I having intermediate frequency is mixed with an in-phase component LO2I of the local signal LO2 having high frequency, and a quadrature-phase path where the quadrature-phase component IF-Q having intermediate frequency is mixed with a quadrature-phase component LO2Q of the local signal LO2 having high frequency.
The signal that passes the in-phase path of the RF mixing unit (203) comprises an in-phase component III having high frequency of the in-phase component II having intermediate frequency and an in-phase component IQI having high frequency of the quadrature-phase component IQ having intermediate frequency. Moreover, the signal that passes the quadrature-phase path of the RF mixing unit (203) comprises an quadrature-phase component QIQ having high frequency of the in-phase component QI having intermediate frequency and a quadrature-phase component QQQ having high frequency of the quadrature-phase component QQ having intermediate frequency.
Signals that pass the in-phase and the quadrature-phase paths of the RF mixing unit (203) are added with each other, and are outputted via the output terminal. The output signal is passed through a power-amplifying unit etc. and is emitted through an antenna.
However, there are many cases that spurious is not rejected fully even if the above quadrature modulating mixer circuit is used in a transmitter. There are mismatch of symmetric circuit or mismatch between in-phase and quadrature-phase signals of local signals synthesized from a frequency synthesizer, and so on, as factors of generating such spurious. In various reasons of generating spurious, the gain mismatch between mixer circuits of the in-phase path and the quadrature-phase path and the phase mismatch between the in-phase and the quadrature-phase components of the local oscillation signal are significant reasons.